1. Field of the Invention
The present invention relates to a flexible printed circuit substrate having a connecting part which is to be attached to a connector.
2. Description of the Background Art
Recently, electronic devices have become lighter and more compact, and flexible printed circuit boards have come into widespread use as materials for electrical circuits in the electronic devices.
A typical flexible printed circuit board has a connecting part which is to be attached to a connector and which is integrally provided on, for example, an edge of the flexible printed circuit board so as to further save space.
FIG. 4(a) is an enlarged perspective view of a connecting part C of a flexible printed circuit board 9a. 
As shown in the figure, the flexible printed circuit board 9a includes a substrate 910 composed of a flexible resin film or the like. The substrate 910 is formed in a predetermined planar shape including the connecting part C which is to be attached to a connector, and a plurality of terminals 93 which are electrically connected to electrical circuits (denoted by reference numeral 92 in FIGS. 4(b) and 5) on the substrate 910 are provided on the surface of the connecting part C. The terminals 93 are arranged in parallel with the direction of the connecting part C being inserted into and extracted from the connector as shown by the blank arrow in the figure.
In FIGS. 4(a) and 4(b), reference numeral 940 denotes a cover-lay for protecting the electrical circuits 92 on the substrate 910 and reference numeral 950 denotes a reinforcing board which reinforces the connecting part C so that the connecting part C can be easily inserted into and extracted from the connector.
The flexible printed circuit board 9a is generally obtained by cutting it out of a flexible printed circuit substrate 9 shown in FIG. 5 by a blanking process or the like, and the flexible printed circuit substrate 9 is manufactured by the processes described below.
With reference to FIG. 5, first, the electrical circuits 92, the terminals 93, and plated leads 93a extending from the ends of the terminals 93, all of which are composed of a metallic thin film of copper or the like, are formed, in predetermined planar shapes by a common lithographic method such as an additive method and a subtractive method, on the surface of a base film 91, which is composed of a resin film or the like and which is to be processed into the substrate 910.
Next, the surface of each terminal 93 is plated with gold, solder, or the like using the plated leads 93a as an electrode.
Next, a protecting film 94, which has an opening 94a for exposing the terminals 93 of the connecting part C and which is to be processed into the cover-lay 940, is laminated on the surface of the base film 91 so as to protect the electrical circuits 92, etc. Although not shown in the figure, the protecting film 94 may also have an opening, as needed, for exposing a mounting space for mounting an element at a predetermined position on the electrical circuits 92.
A thick reinforcing film 95, which is to be processed into the reinforcing a board 950, is laminated on the opposite surface, that is, the back surface of the base film 91. Thus, the flexible printed circuit substrate 9, which is to be processed into the flexible printed circuit board 9a, is manufactured.
The thus manufactured flexible printed circuit substrate 9 shown in FIG. 5 is cut by blanking along the peripheral edge F shown by the dot-dashed line which defines the planar shape of the flexible printed circuit board 9a in the figure. In addition, an element is mounted in the above-described mounting space if necessary. Accordingly, the flexible printed circuit board 9a shown in FIG. 4(a) is manufactured.
Such blanking may be at once or part-by-part along its peripheral edge F, using a blanking mold, for example.
In the above-described blanking process, the blanking mold is normally positioned by inserting guide pins provided on the blanking mold through positioning holes formed in the base film 91 at predetermined positions outside the region shown in the figure so that the position of the blanking mold may be adjusted with respect to the terminals 93, etc., provided on the base film 91.
However, since the positioning accuracy achievable in the blanking process is limited, a connection failure may be caused such that the terminals 93 cannot be precisely connected to their respective contacts in the connector when the connecting part C of the flexible printed circuit board is attached to the connector.
The connector and the connecting part C are normally positioned with respect to each other by bringing the sides F1 and F1 of the connecting part C, which are formed in parallel with the insertion/extraction direction shown by the blank arrow in FIG. 4(a), into contact with guides provided on the connector so that the terminals 93 are precisely connected to their respective contacts.
Therefore, the positioning accuracy between the connector and the connecting part C depends on the dimensional accuracy of the pitch (p1 in FIG. 4(b)) between one of the two sides F1 and the centerline of the terminal 93 that is nearest to this side F1, for example. That is, the positioning accuracy between the connector and the connecting part C depends on the absolute value of ±α in the formula:p1′=p1±α  (1),where p1′ is the actual pitch, p1 is the desired pitch, and ±α is the error between them,and the dimensional accuracy ±α of the pitch p1 depends on the positioning accuracy in the above-described blanking process.
However, the positioning accuracy achievable by the positioning holes and the guide pins is relatively low, and the deviation is more than about ±100 μm (±0.1 mm).
Accordingly, there is the possibility of a connection failure when the connecting part C is attached to the connector in the case where the pitch (p2 in FIG. 4(b)) between two adjacent terminals 93 of the connecting part C is reduced to, for example, about 0.5 mm for obtaining lighter and more compact electronic devices as described above.
In order to surely prevent the connection failure of the flexible printed circuit board in which the pitch p2 between two adjacent terminals 93 is about 0.5 mm, methods for improving the positioning accuracy in the blanking process of the flexible printed circuit substrate are studied.
For example, Japanese Patent No. 2662477 discloses a construction in which a target mark composed of a metallic thin film is formed along with terminals 93, etc. at a position near the connecting part C on a flexible printed circuit substrate 9, and blanking (trimming) for cutting the sides F1 of the connecting part C is performed while positioning a trimming punch (multipunch) by detecting the center of the target mark with a specific measuring camera.
According to this method, a positioning accuracy of ±100 μm or less can be obtained in the blanking process and the connection failure can be surely prevented in the case of the flexible printed circuit board 9a in which the pitch p2 between two adjacent terminals 93 is set to about 0.5 mm to achieve microconnection.
However, further reduction in the pitch of the electrical circuits 92 on the flexible printed circuit board 9a is required for making the electronic devices more lighter and compact, and accordingly, the pitch p2 between the terminals 93 of the connecting part C is also required to be reduced still further.
More specifically, although the pitch p2 between adjacent terminals 93 for microconnection was conventionally 0.5 mm, connecting parts having a pitch p2 of about 0.3 mm have become common, and those with a pitch p2 of about 0.2 mm have been developed recently.
However, the accuracy obtained by the positioning method according to Japanese Patent No. 2662477 is limited to about ±70 μm, and further improvement in the positioning accuracy is difficult in the method.
Therefore, the positioning accuracy cannot be improved enough to comply with further reduction in the pitch, and the connection failure which occurs when the connecting part C of the flexible printed circuit board 9a is attached to the connector has become a problem again.